The major source of feedback for control of breathing comes from central respiratory chemoreceptor that monitor blood CO2 levels. Dysfunction of these neurons occurs in many common diseases, including chronic obstructive pulmonary disease (COPD), sleep apnea, and possibly sudden infant death syndrome (SIDS). The first step in finding specific treatments for these diseases is to identify the neurons responsible for chemoreception, and define their mechanisms. Although the central chemoreceptors were localized to the ventrolateral medulla (VLM) 40 years ago, the specific neurons responsible have still not been clearly identified. We recently obtained evidence that serotonin-containing neurons within the VLM are central respiratory chemoreceptors, but the majority of neurons with identical properties are located in the medullary raphe. This is exciting, because chemosensitivity of serotonergic neurons could provide a biological basis for the interaction between sleep and breathing. The proposed work is aimed at further defining the cellular mechanisms of these neurons, and the role that they play in central chemoreception. We propose to use a combination of patch clamp recordings from neurons in tissue culture and brain slices, imaging of intracellular pH, immunohistochemistry, confocal microscopy, and computer modeling to address basic unanswered questions about chemosensitive raphe neurons. 1) Do medullary raphe neurons have properties that would make them uniquely specialized to sense changes in blood CO2? We will look at their anatomical relationship with blood vessels, the co-transmitters they contain, and their projections. 2) Are there differences between chemosensitive neurons in the medullary raphe and the VLM? 3) Does chemosensitivity of midbrain raphe neurons explain the arousal that occurs in response to hypercapnia during sleep? 4) What ion channels are responsible for chemosensitivity? 5) Does CO2 act through a change in intracellular pH alone? 6) Can the depression of breathing during sleep be explained in part by the effects of reticular activating system neurotransmitters on raphe neurons? Disturbances of breathing are common in human diseases, particularly during sleep. Understanding the basic mechanisms involved in modulation of neuronal activity by CO2, and the mechanisms by which breathing is affected by sleep, may help provide successful treatment for these diseases.